1. Field of the Invention
The invention relates to the field of controlling the application of power to electrical loads, and in particular concerns a controller operable instantaneously to insert a high resistance between a load and a power line, when breaking the circuit between the load and the line.
2. Prior Art
Load controllers, such as motor starters and similar circuit breaking contactors, typically comprise a relay that upon energizing of the relay coils, closes an electrical circuit coupling a load to a power source. Often, the controller is equipped with an overload protector which opens the relay to disconnect power from the load upon occurrence of a fault condition. The fault conditions triggering opening of the circuit may be an instantaneous overcurrent or undervoltage on the line, or an average current load that exceeds a predetermined limit. Opening of the contactor relay prevents flow of excessive current that may damage either the line or the load.
The timing of the circuit breaking operation is important. In the event of a direct short circuit which sinks a very large current, the relay may be too slow to open in time to prevent damage to the load or line. Furthermore, electrical arcs associated with inductive loads or very excessive currents can weld the relay contacts shut, preventing the safety features of the load controller from opening the relay at all. Even if the relay contacts are able to open, arcing across the relay contacts erodes them. Pitting and accumulation of carbon deposits impede current flow during normal use, and contribute to further arcing in the event of a subsequent protective circuit breaking operation.
The present invention applies conductive polymers to the environment of a protective circuit breaking contactor. Certain polymers have a relatively low electrical resistance when the current through the polymer is low, and when the current increases to a high level, such polymers change state very rapidly on the molecular level, and exhibit a very high electrical resistance. The high resistance state may be tens to hundreds of times greater than the nominal, low resistance state at the lower current level. This characteristic can be applied effectively to limit current passing through the polymer.
There is a need to protect loads and lines from damaging short-circuit currents, effectively and efficiently, and also to provide basic fault current protection. The combination controller according to the invention achieves this in a load controller having a relay for opening a circuit to a load, with a conductive polymer in series with the relay contacts. The electrical resistance of the conductive polymer increases substantially immediately upon application of a high load current level, thereby inserting protective electrical resistance in series with the load. This combination load controller protects the line and the load from high short-circuit currents, decreases arcing across the relay contacts, and more dependably operates to open the circuit between the line and the load.